The present invention relates in general to detecting and reducing the effects of intermodulation distortion in a radio receiver, and more specifically to maintaining intermodulation distortion below a predetermined distortion while maintaining the level of a desired channel signal to above a predetermined level.
A persistent problem in the art of radio reception has been the creation of intermodulation distortion by radio receivers. Whenever two electrical signals having different frequencies are applied to any nonlinear device, mixing of the signals gives rise to mixing products at the sum and difference frequencies of the original signals. In a radio receiver, it is impractical to produce amplifiers and other components which are completely linear. Furthermore, other components, such as filters, depend on nonlinearities for their operation.
Whenever a receiver is receiving at least two RF broadcast signals within its RF bandwidth, mixing products at the sum and difference frequencies can potentially be created which propagate through the receiver with the broadcast signals. Whenever the two frequencies are close in frequency and large in signal strength, it is possible that the mixing products will include frequencies spanning a desired RF broadcast signal at a third frequency. The mixing products add to the desired RF broadcast signal giving rise to what is known as intermodulation distortion or intermodulation interference.
Most radio receivers have been equipped with an RF front end having a fixed bandwidth sufficiently wide to accept the entire RF broadcast band which it will receive. One technique which has been used to reduce the susceptibility of radio receivers to intermodulation distortion is to instead use a tunable RF filter having a bandwidth narrower than the entire RF band. By properly tuning the resonant frequency of the tunable RF filter, a desired RF signal is received while other signals in the particular broadcast band are rejected. This improves selectivity, image rejection, and intermodulation performance of the receiver. Nevertheless, it is not feasible to construct tuned RF filters of a sufficiently narrow passband to eliminate all signals other than the desired signal (i.e., having a passband with the width of a single channel or even just a few channels). This is especially true for the FM broadcast band due to its high frequencies in the range of from 88 MHz to 108 MHz.
Intermodulation distortion can also be favorably reduced through use of the automatic gain control (AGC) circuit which is common in radio receivers. An AGC control signal is generated which controls the forward gain of the radio signal amplification. When the gain applied to the RF signals is reduced, the intermodulation products are reduced to a greater degree. However, the improvement in intermodulation distortion is often obtained at the expense of overly-reducing the desired signal.
The AGC control signal has been generated in dependence on the received signal strength in the received RF frequency band, i.e, a plurality of channels not limited to the desired signal. Since the amount of signal strength in the desired signal versus signals in other channels which could cause intermodulation distortion is unknown, an AGC control signal can be generated based only on prior approximations of intermodulation distortion likely to be encountered.
It has been proposed to infer the presence of intermodulation distortion by measuring RF signal strength in various channels surrounding the desired signal. For example, in Sakai et al U.S. Patent No. 4,654,884, a radio receiver is disclosed which performs a scan tuning operation in search of any nondesired channels having a strong received signal therein. Based on the known frequency of the desired channel and the frequencies of any nondesired channels which are identified having a strong signal therein. The radio receiver then examines the signal strength at those nondesired frequencies that when paired with the strong signals would create intermodulation distortion in the desired channel. Thus, the presence of intermodulation distortion is inferred if conditions exist for creating it. However, this technique is unreliable since intermodulation distortion is not measured directly. Either two separate receivers or interruption of the reception of the desired signal is required. Furthermore, this technique results in a large increase of complexity and cost in the radio receiver.